This invention relates generally to tools and methods for measuring the force applied to a surface and more particularly to a tool that is capable of simultaneously measuring both the total force applied to the surface and the centroid of the force so applied and to a method of using the measured total force and centroid to alter either or both the measured total force and the position of the centroid of the force.
It is well known that semiconductor devices must be and are extensively tested. Many such tests are usually performed after the semiconductor device has been partially completed. One such test is the process known as semiconductor device test and burn-in. This process is a widely known and used testing process for measuring the expected life of a semiconductor device. Typically the semiconductor device is tested after the semiconductor chip is mounted on a module base but before a module cap has been applied over the chip. To perform this test and burn-in, respective uncapped semiconductor devices are placed in respective sockets on a so-called Burn-In-Board (BIB) provided with a plurality of such sockets. The filled Burn-In-Board is then placed in an oven and heated while being electrically connected to a suitable test apparatus so that the semiconductor devices can be tested while being subjected to various electrical and temperature conditions.
Because the operation of the semiconductor device can, in itself, produce heat in the chip and because of convection currents in the oven, the temperature gradients across the surface of any one each chip can vary significantly from those across the surface of any other chip although all the chips are identical and are being subjected to the same electrical and/or oven temperature conditions. Without assurance that each chip is meeting the same temperature conditions, only an estimate of what can be expected in actual operation can be realized. Therefore, it is desirable that the temperature range and distribution across the entire surface of each chip under going the test be known, be closely controlled. To try and control the temperature range and distribution across the surface of each chip under going the test the prior art placed heat sinks against the surface of the chips.
It has been recognized that thermal contact between the heat sink and the device under treatment will be improved when increased force is applied to the device by the heat sink. However, increasing the force beyond certain limits may cause damage to the device without improving the temperature distribution or gradient. Therefore a means to measure and optimize the force applied by the heat sink to the chip has long been sought by the prior art.
Today, the presently available, state of the art burn-in equipment attempts to solve these problems by using substantially planar heat sinks that are provided with spring loading and gimbal action. Such heat sinks may also be coupled, for example to a microprocessor temperature controller, a temperature sensing means, heating and cooling means. Heat sinks similar to those used in Burn-In-Boards are also used in module handlers in which the semiconductor may under go additional more extensive electrical tests one module at a time.
Although the start of the art heat sinks have provided improved results, there still is no assurance that all the devices being tested are subjected to identical conditions even when an optimum force is applied, by the heat sink, to the chip.
It has now been found, by the present inventors, that failure of the prior art to solve the above problem occurs because each respective heat sink in the burn-in board can fail to be positioned correctly with respect to the entire surface of its respective chip. For example even with the spring loaded, gimbal action heat sinks of the prior art, a heat sink may be improperly positioned in a socket such that it is not in contact with the surface of the underlying chip or in contact with only a single corner or edge of the chip surface. The result of this inaccurate positioning of the heat sink with respect to the surface of the chip will be large temperature gradients across the chip surface. Another result, for example, may be that the microprocessor controller will be unable to maintain proper chip temperature.
This inaccurate positioning of the heat sink can be caused, for example, by a twist in one of the electrical connectors or in the cooling or heating supply lines connected to the heat sink which can be so slight that a visual inspection would not uncover any error in the position of the heat sink.
The present inventors have now discovered that good thermal contact between the chip under test and the heat sink is a function of both the total force applied by the heat sink to the chip and the position of the centroid of the applied force with respect to the center of the chip.
The present invention is thus directed to a mechanism that will consistently and accurately provide for and permit the proper positioning of a heat sink with respect to a semiconductor device in a test socket by measuring not only the total amount of force applied by a heat sink to an underlying surface but also by measuring the centroid of the applied force, i.e., total amount of force applied by the heat sink, with respect to the center of the underlying surface.
With these measurements, the position of the heat sink, in the socket, can be adjusted until that the total force applied by the heat sink is substantially equal to the desired applied force and that the centroid of the applied force is substantially aligned with the center of the underlying chip surface.
The present invention is thus directed to a mechanism capable of simultaneously measuring both the total force applied to the surface and the centroid of the force so applied between a heat sink and an underlying chip surface.
The present invention is also directed to a method of measuring both the total force applied to the surface of a semiconductor device and the centroid of the applied force and repositioning the heat sink so that the total applied force and/or its centroid can be adjusted such that the total applied force is within a desired range and that the centroid of the applied force is substantially aligned with the center of the surface of the semiconductor chip.
More particularly, the present invention is an assembly, including an upper and a lower plate with a plurality of load cells therebetween for measuring the applied force and determining its centroid, with respect to the center of the surface of the upper plate.
If the surface of the heat sink is not substantially parallel with the surface of the upper plate, the centroid of the force, applied by the heat sink, will cause the amount of load registered by at least one of the three load cells to be different from that registered by the other two load cells. In such an instance the position of the heat sink is adjusted until the registered load of each of the load cell becomes substantially equal. Substantial equality between the load cell measurements means that the centroid of the force applied by the heat sink is substantially positioned over the center of the assembly. At this time the heat sink and assembly can be removed from the Burn-In-Board socket. A semiconductor device, having a chip thereon whose center is substantially identical to the center of the top plate of the assembly, is now substituted, in the socket, for the assembly and the heat sink applied to the surface of the semiconductor device. Because the heat sink is replaced in the socket, over the semiconductor device in exactly the same position as it was prior to the removal of the assembly, one can be assured that the heat sink will apply substantially the identical force to the semiconductor device as it applied to the assembly and that the centroid of the applied force is aligned with the center of the semiconductor device substantially the same as it was applied to the surface of the assembly.
The bottom plate of the assembly is positioned in a semiconductor device socket so that the top plate of the assembly will be in the same position as the surface of the chip mounted on a semiconductor device will be when the semiconductor device is placed in the socket. Thus the heat sink will contact the chip surface exactly as it contacted the top plate of the assembly thus assuring that the force in a manner identical to the semiconductor chip surface so that the force applied by the heat sink against the top plate of the assembly will be measured by the 3 load cells permitting the location of the center of the force to be determined with respect to the center of any semiconductor device later positioned in the socket.
The present invention thus provides a means of improving uniformity of the thermal contact between each heat sink and the semiconductor device it is in contact with regardless of the thermal interface used.
These objects, features and advantages of the present invention will become further apparent to those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings wherein: